The objectives of this project are to more clearly elucidate the mechanisms of ventilatory control during acclimatization to chronic hypoxia, and to determine what role the peripheral chemoreceptors have in this acclimatization. Additionally we wish toestablish more adequately what functions the peripheral chemoreceptors have in normal resting ventilation at sea level as well as at high altitude. The ventilatory control system during exercise in normoxia and chronic hypoxia will also be examined. The investigation will be carried out in ponies. Measurements of resting ventilation and the ventilatory response to acute isocapnic hypoxia, acute hypercapnia, and doxapram hydrochloride infusion will be made. Cerebrospinal fluid pH, PCO2, lactate and norepinephrine metabolite analysis and blood acid-base analysis will be carried out. All measurements will be done before and after carotid body excision, aortic body excision, both carotid and aortic body excision and during acclimatization to chronic hypoxia in normal and totally chemodenervated animals. Similar studies will also be done during exercise.